Determining the position of rotary devices in work machines both during operation and at start-up may enable better control of the devices. For example, knowing the position of a crankshaft in an engine, and thus top dead center in engine cylinders, may enable more effective engine controls. Some types of motors and generators, such as switched reluctance and permanent magnet, control the relationship of the rotor to stator fields. Knowing the position of the rotor is necessary to this type control. Knowing the speed of a rotor may also be necessary and/or desirable.
U.S. Pat. No. 5,773,820 to Osajda discloses a steering wheel shaft position sensor including a light emitter fixed with respect to the shaft and a pair of light detectors fixed with respect to the emitter. A disk is mounted for rotation on the shaft between the emitter and the detectors and has a grey scale track extending circumferentially on the disc between the emitter and one of the detectors. The grey scale varies from opaque to transparent to the light emitted by the emitter. A second, transparent track extends circumferentially on the disc between the emitter and the other detector. The outputs of the two detectors are compared to provide an indication of the movement of the shaft.